Tow car MPG: True MPG special

Caravanning can be a low-cost holiday option, but fuel economy inevitably suffers when your car has to pull a 1.5-tonne white box. The same applies to owners of horseboxes and any other kind of trailer – so just how much does towing hurt a car's fuel efficiency?

To find out, we sent members of our True MPG team to the annual Tow Car Awards tests, where they assessed the fuel economy of four cars with and without a caravan attached.

Each was hitched to the same Swift caravan at 85% of the car's kerbweight.

We chose the Volkswagen Golf 2.0-litre TDI as a typical front-wheel-drive diesel hatch, while we also tested the Mazda 6 2.0-litre saloon to find out if average economy of petrol cars is hit harder than it is for diesels when towing.

Smaller 4x4s were represented by the Dacia Duster, while the Land Rover Discovery was there to test the theory that fuel economy of heavier, more powerful cars holds up better when hauling a trailer.

Volkswagen Golf 2.0 TDI GT DSG 5drFirst, we drove it into town to assess its urban economy, then took an A-road and motorway route for the extra-urban test. Without a caravan, the Golf set benchmark figures of 40.6mpg for town driving and 53.6mpg on the open road, with an average economy of 46.2mpg.

Heading back into town with the caravan, the 2.0 TDI engine felt strong, easily coping with the additional weight it was hauling. There was no sense of having to work the engine hard to drag the car and its load up to speed, and the DSG automatic transmission changed gears with unobtrusive efficiency.

On faster roads, the Golf felt stable and controlled. Again, although the engine was working harder now, it didn't seem a struggle to pull the caravan up to 60mph on the motorway.

With the caravan, urban economy was down to 27.1mpg and extra-urban to 23.7mpg. The average economy was 25.3mpg, a drop of 45%. That means a 500-mile journey when towing would cost £127.40 in fuel.

Mazda 6 2.0 165 SportWith less mid-range pulling power, petrol engines usually have to work harder than their diesel equivalents to keep up with traffic when towing. This should mean a bigger decline in economy when towing.

Around town and with the caravan, the Mazda 6 saloon needed a heavier right foot than the Golf when pulling out at junctions and roundabouts. It's not that the 6 felt underpowered, but more revs were needed to persuade car and load to shift with any urgency.

On faster roads, more gearchanges were required to maintain speed on hills – with each shift taking a toll on fuel consumption. However, the 6 felt comfortable at a steady 60mph on the motorway, with no sense that the engine was straining.

The 6 posted impressive economy figures in regular driving: 44.1mpg around town, 51.0mpg on the open road and average economy of 47.3mpg – slightly better than that of the diesel Golf.

However, towing reduced those figures to 23.0mpg around town, 22.3mpg in the country and an average economy of 22.7mpg. That's a huge 52% fall in economy, giving a fuel bill of £137.08 for a 500-mile trip with a caravan in tow.

As with the VW and Mazda, we drove the routes without a caravan first. For each test the car's four-wheel-drive system was left in auto, so despite it being a 4x4, power was going to the front wheels unless the car struggled for traction.

Around town, the Dacia achieved 39.9mpg, on A-roads and motorways it returned 48.8mpg with average economy of 43.9mpg.

With the caravan hitched, the Duster's 1.5-litre engine didn't need as many revs as the Mazda's 2.0 petrol to build speed, although you wouldn't know it from the volume of engine noise in the cabin – even at a steady cruise on A-roads and motorways. Otherwise, though, it was a capable tow car.

Once the data was processed, the Duster looked even better; returning 28.1mpg around town, 27.4mpg on the open road and average economy of 27.8mpg. That's just 37% down on its caravan-less economy – which, at £115.94, is the lowest 500-mile fuel cost of all four cars tested.

Land Rover Discovery 3.0 SDV6 HSEThe Discovery was an overall winner at the Tow Car Awards two years ago, so we had no doubt it would haul well, but what about its fuel economy?

Without a caravan, the Discovery achieved 24.6mpg in town and 29.6mpg on the out of town route. The average economy figure was 26.9mpg. As expected, given its bulk, it was much thirstier than the other cars on test.

Hitch a caravan, though, and the Discovery was totally unfazed. The 3.0 engine has so much pulling power that even an unwieldy load such as this seemed to make little difference to its performance. Whether dealing with traffic on the urban route, or holding speed uphill on an A-road, the Discovery made towing seem easy. From the driver's seat, the engine didn't seem to be working hard at all.

The overall impression was that economy suffered less when towing with the Land Rover than with any of the other vehicles. The data backed this up.

Fuel economy dropped to 17.9mpg in town and 18.0mpg out, giving average economy of 18.0mpg. That's 33% down – the lowest figure of all four cars. However, the 500-mile fuel bill of £179.07 is, unsurprisingly, the steepest of the four here.

What Car? says…We expected a big, powerful car to have enough muscle to make hauling a caravan less of a strain, and so it was. The fuel economy of the biggest of our quartet was affected least by towing, but if you want the best economy a small diesel is the best compromise.

The Dacia Duster's average economy of 27.8mpg is nearly 10mpg better than the Discovery's, a saving of £63.13 over 500 miles.

The petrol Mazda 6 seemed to be working hardest when towing, and the True MPG data backed that up. The 6's fuel economy fell by a huge 52%. Without a caravan, however, it was the most economical car here.